126 Dr. Brewster on the laws which regulate the 
“ dispersive forces. It is a property of bodies independent 
“ of the other modes of action which they exercise upon 
“ light/' 
This premature generalisation of a few imperfectly ascer- 
tained facts, is perhaps equalled only by the mistake of Sir 
Isaac Newton, who pronounced the construction of an 
achromatic telescope to be incompatible with the known prin- 
ciples of optics. Like Newton, too, Malus himself aban- 
doned the enquiry ; and even his learned associates in the 
Institute, to whom he bequeathed the prosecution of his views, 
have sought for fame in the investigation of other properties 
of polarised light. 
In the summer of 1811, when my attention was first turned 
to this subject, I repeated the experiments of Malus, and 
measured the polarising angles of a great number of trans- 
parent bodies. I endeavoured, in vain, to connect these results 
by some general principle : the measures for water and the 
precious stones afforded a surprising coincidence between the 
indices of refraction and the tangents of the polarising angles ; 
but the results for glass formed an exception, and resisted 
every method of classification. Disappointed in my expecta- 
tions, I abandoned the enquiry for more than twelve months, 
but having occasion to measure the polarising angle of topaz , 
I was astonished at its coincidence with the preceding law, 
and again attempted to reduce the results obtained from glass 
under the same principle. The piece which I used had two 
surfaces excellently polished. The polarising angle of one 
of these surfaces almost exactly accorded with the law of the 
tangents, but with the other surface there was a deviation of 
no less than two degrees. Upon examining the cause of this 
